1. Field of the Invention
The present invention relates to a method of bonding a conductive adhesive and an electrode together, and the bonded structure obtained thereby. More, particularly, the present invention relates to a method of electrically and mechanically bonding a surface mount device to a land electrode using a conductive adhesive and the resulting bonded structure.
2. Description of the Related Art
Surface mount devices are sometimes bonded to a land electrode on a mother substrate using a conductive adhesive in place of solder. The conductive adhesive comprises an organic binder (e.g., a thermosetting epoxy resin) and a conductive filler (e.g., Ag particles).
In order to mount the surface mount device on the land electrode, the surface mount device is adhered to the land electrode with the conductive adhesive provided therebetween, and then both components are pressure-bonded together and heated to cure the conductive adhesive so as to electrically and mechanically bond the surface mount device to the land electrode. As a result of this process, the conductive filler particles contained in the conductive adhesive are connected to one another in a form of chain to produce a conductive path, thereby obtaining conductivity between the land electrode and the surface mount device.
While the foregoing process results in a secure bond between the surface mount device and the land electrode, the conductive adhesive absorbs moisture which will cause a defective bonding at the interface between the conductive adhesive and the land electrode when the bonded structure is allowed to stand in a high-humidity atmosphere in a pressure cooker test or the like, thereby causing an increase in the electric resistance of the bonded portion. For example, when the pressure cooker test is performed for 100 hours, the resistance value is at least ten thousands times as large as the initial resistance value.
Japanese Unexamined Patent Publication Application No. 7-179832 discloses a method of achieving bonding with high reliability using such a conductive adhesive. The method comprises forming low-melting-point metallic layers on the surfaces of the conductive filler particles, curing a binder resin contained in the conductive adhesive and, and at the same time, melting the low-melting-point metallic layers on the surfaces of the conductive filler particles to fusion-bond the conductive filler together. As a result, bonding with high reliability can be obtained due to fusion-bonding of the metal particles of the conductive filler.
This prior art structure has two major drawbacks. It is costly because of the need to utilize a special conductive filler wherein low-melting-point layers are formed on the surfaces of the metal particles. Additionally, it does not address the problem of connection reliability at the interface between the conductive adhesive and the land electrode.
Accordingly, a main object of the present invention is to provide a method of bonding a conductive adhesive and an electrode together which is capable of achieving good electrical bonding between the conductive adhesive and the electrode.
Another object of the present invention is to provide a bonded structure having high reliability and good electrical bonding.
In accordance with an aspect of the present invention, there is provided a method of bonding a conductive adhesive and an electrode together comprising:
coating a conductive adhesive containing a conductive filler and an organic binder on at least a surface of an electrode formed on a substrate; and
melting at least the surface of the electrode to form a fusion-bond between the surface of the electrode and the conductive filler; and
curing the organic binder contained in the conductive adhesive to electrically and mechanically bond the conductive adhesive and the electrode together.
In this method, the surface of the electrode is melted and fusion-bonded to the conductive filler, and thus the metal particles of the electrode surface and the conductive filler are fusion-bonded together with the conductive filler entering the surface of the electrode, thereby bonding the conductive adhesive and the electrode together with high reliability and good electrical properties.
In the method of bonding the conductive adhesive and the electrode of the present invention, a thermosetting resin is preferably used as the organic binder, and thus heating the conductive adhesive and the electrode under predetermined heating conditions causes the surface of the electrode to be melted and fusion-bonded to the conductive filler contained in the conductive adhesive and causes the organic binder to be thermally cured to bond the conductive adhesive and the electrode together.
The surface of the electrode preferably comprises a low-melting-point metal material which can be melted under the above heating conditions.
In order to obtain the surface of the electrode, a layer can be formed on the surface of the electrode by plating a low-melting-point metal material.
The conductive filler contained in the conductive adhesive preferably comprises scale-like conductive filler particles. The scale-like conductive filler particles easily enter the surface of the electrode, thereby enhancing the bonding strength between the conductive adhesive and the electrode.
When the surface of the electrode is melted, at least a portion of the conductive filler contained in the conductive adhesive is also melted. As a result, the conductive adhesive and the electrode are bonded together, and the respective conductive filler particles are fusion-bonded together to decrease the resistivity of the conductive adhesive, thereby obtaining excellent electrical properties.
In accordance with another aspect of the present invention, there is provided a method of bonding a pair of electrodes having a conductive adhesive, the method comprising:
coating the conductive adhesive containing a conductive filler and an organic binder on at least a first electrode formed on a surface of a first substrate;
adhering the conductive adhesive formed on the first electrode to a second electrode formed on a surface of a second substrate;
melting at least the surfaces of the first electrode and the second electrode to form a fusion-bond between the surfaces of the first and second electrodes and the conductive filler contained in the conductive adhesive;
and curing the organic binder contained in the conductive adhesive to electrically and mechanically bond the conductive adhesive and the first and second electrodes together.
In this method, the surfaces of the first and second electrodes are melted and fusion-bonded to the conductive filler, and thus the metal particles of the surface of each of the electrodes and the conductive filler are fusion-bonded together with the conductive filler entering the surfaces of the electrodes. As a result, the first and second electrodes are bonded together via the conductive adhesive with high reliability and good conductivity.
In accordance with a further aspect of the present invention, there is provided a bonded structure formed by any one of the above-described methods of bonding a conductive adhesive and an electrode. In this bonded structure, the conductive filler contained in the conductive adhesive and the electrode are fusion-bonded together with the conductive filler entering the surface of the electrode, thereby achieving good electrical bonding with high reliability.